Add some missing Dispose calls, fix some Dispose implementations.

OpenRA.Mods.Common/LoadScreens/LogoStripeLoadScreen.cs

@@ -79,12 +79,12 @@ namespace OpenRA.Mods.Common.LoadScreens
 			r.EndFrame(new NullInputHandler());
 		}
 
-		public override void Dispose()
+		protected override void Dispose(bool disposing)
 		{
-			if (sheet != null)
+			if (disposing && sheet != null)
 				sheet.Dispose();
 
-			base.Dispose();
+			base.Dispose(disposing);
 		}
 	}
 }